The apparatus herein disclosed is designed to monitor and keep constant the density of a pressurized sulfur hexafluoride gas within large outdoor circuit breakers utilized by power companies. The gas is used as an arc-suppressant upon opening of the breaker contacts for the high pressure system and as an electrical insulator to prevent any current drain to the outer walls or any other grounded parts of the circuit breaker in the low pressure system. Because the dialectic strength of the gas is a function of the density, it is desirable and required that the latter must be kept constant. Since the density is a function of pressure, temperature and volume, these parameters must be dealt with in this type of control. According to the invention there are two models involved, one for high and one for lower pressure system. In both systems micro-switches or valves are used as disclosed in U.S. Pat. No. 3,490,342.